The present invention relates to an optical multilayer structure having a function of reflecting, transmitting or absorbing incident light, a method of manufacturing the same, an optical switching device and an image display apparatus.
In recent years, displays as image information display devices have been taken on greater importance, and the development of optical switching devices (light valves) operating at high speed as devices for the displays and devices for optical communications, optical storage, optical printers or the like has been in demand. Conventionally, as devices of this kind, a device using a liquid crystal, a device using a micromirror (DMD: digital micromirror device, a trademark of Texas Instruments Inc.), a device using a diffraction grating (GLV: grating light valve manufactured by SLM (Silicon Light Machines)) and so on are cited.
The GLV forms a diffraction grating with MEMS (micro-electromechanical systems) to implement an optical switching device with a high speed of 10 ns by an electrostatic force. The DMD has the same MEMS to perform switching through moving a mirror. A display such as a projector can be implemented by the use of these devices, however, in order to implement the display as a light valve, a two-dimensional array must be used because a liquid crystal and the DMD have low operating speed, thereby the configuration of the display becomes complicated. On the other hand, the GLV is a high-speed operation type, so a projection display can be implemented through scanning a one-dimensional array.
However, the GLV has a diffraction grating, so there is complexity such that a pixel is required to include six devices, or diffracted lights directed toward two directions are required to be focused on one direction by an optical system of some kind.
A light valve which can be implemented with a simple configuration has been disclosed in U.S. Pat. No. 5,589,974 or U.S. Pat. No. 5,500,761. The light valve has such a configuration that a translucent thin film with a refractive index of √{square root over ( )}ns is disposed on a substrate (with a refractive index ns) with a gap portion (gap layer) in between. In the device, the thin film is driven by the use of an electrostatic force to vary a distance between the substrate and the thin film, that is, a size of the gap portion, and thereby an optical signal is transmitted or reflected. Herein, the refractive index of the thin film is √{square root over ( )}ns with respect to the refractive index ns of the substrate, and it is considered that high-contrast optical modulation can be achieved through satisfying such a relationship.
However, in the device with the above-described configuration, there is such a problem that unless the refractive index ns of the substrate is as large a value as “4”, high-contrast optical modulation cannot be achieved in a visible light range. In other words, the translucent thin film requires strength as a three-dimensional configuration, so a material such as silicon nitride (Si3N4) (refractive index n=2.0) is preferable as the translucent thin film, however, in this case, the refractive index ns of the substrate is 4. In the visible light range, there are not many choices of materials of this kind. In a wavelength for communications such as infrared radiation, the device can be implemented by the use of germanium (Ge) (n=4), silicon (Si) (n≦4) or the like.
Moreover, in the light valve disclosed in U.S. Pat. No. 5,500,761 or the like, a structural material such as silicon nitride (Si3N4) is disposed on a silicon substrate with a gap portion in between. However, in such a configuration, a widely recognized method as dry etching of a sacrificial layer, that is, a method of forming a sacrificial layer of silicon (Si), and then etching the sacrificial layer by the use of xenon difluoride (XeF2) cannot be used. It is because, as the substrate is also formed of Si, selectivity to the sacrificial layer cannot be obtained. Therefore, in the above-described configuration, it appears that other method such as wet etching is used. In wet etching, it is difficult for an etching solution to enter into and smoothly circulate in a gap of approximately λ/4, so etching may not be able to be performed, or a structural material on the sacrificial layer may be damaged during drying due to a surface tension of the solution or the like, thereby it is difficult to form a desired structure. When a relative area of a part which becomes an optical switch is reduced so as to decrease an aperture, the structure can be implemented through a process such as wet etching, although, in a structure for image display, there is a tendency to increase the aperture, so it is desired to reduce a portion where the etching solution enters. Therefore, dry etching is more suitable than wet etching. However, as described above, when the substrate is formed of Si, there is such a problem that a method of forming the sacrificial layer of Si, and then etching the sacrificial layer by XeF2 is not applicable.
In view of the foregoing, it is a first object of the invention to provide an optical multilayer structure which has a simple, compact and lightweight configuration and flexibility in selection of a structural material, is capable of high-speed response in a visible light range, and is preferably used for an image display apparatus or the like.
It is a second object of the invention to provide a method of manufacturing an optical multilayer structure in which in a step of forming a gap portion, even though a sacrificial layer is formed of Si, dry etching using XeF2 is applicable, and which is capable of easily manufacturing an optical multilayer structure with high aperture.
Moreover, it is a third object of the invention to provide an optical switching device and an image display apparatus which are capable of high-speed response by the use of the above optical multilayer structure.